Memory device with page buffer having dual registers and method of using the same

ABSTRACT

A nonvolatile memory device and programming method and apparatus therefore are described that include operatively coupled first and second sense amplifiers having first and second data registers or latches, a storage circuit for storing a data of the second amplifier, a pass/fail check circuit for checking the content of the second data register whether a cell of the memory device has been sufficiently programmed and a restore circuit for resetting the second data register for reprogramming the device until sufficiently programmed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/284,604, filed Nov. 21, 2005, now pending, which is a continuation ofU.S. patent application Ser. No. 11/153,638, filed Jun. 14, 2005, nowU.S. Pat. No. 6,996,014, which is a continuation of U.S. patentapplication Ser. No. 10/315,897, filed Dec. 9, 2002, now U.S. Pat. No.7,042,770, which is a continuation-in-part of U.S. patent applicationSer. No. 10/013,191, filed Dec. 7, 2001, now U.S. Pat. No. 6,671,204,which claims priority from U.S. Provisional Application No. 60/307,572,filed Jul. 23, 2001, all of which are hereby incorporated by referencein their entirety.

BACKGROUND

1. Technical Field

This disclosure relates to the field of semiconductor memory devicesand, more specifically, to a flash memory device with a page buffercircuit having dual registers.

2. Description of the Related Art

The recent trends in semiconductor memory devices are for highintegration, large capacity, and to support systems operating at highspeeds. These trends are for both volatile memories (e.g., DRAM andSRAM) and non-volatile memories (e.g., flash memories).

Flash memories are generally subdivided into NOR-type flash memories andNAND-type flash memories. The NOR-type flash memories are used inapplications necessary for reading information of a low volume at a highspeed non-sequentially, while the NAND-type flash memories are used inapplications necessary for reading information sequentially.

Flash memory devices use memory cells to store data. The memory cellsinclude cell transistors. Each cell transistor has a control electrodeand a floating gate. Since the flash memory device stores informationusing tunneling via an insulation film, it takes some time to storeinformation.

In order to store information of a large volume in a short time, theNAND-type flash memory uses a register, which is also known as a pagebuffer circuit. Large volumes of data are supplied externally, for quickstoring in the storage region. They are first stored in the register,and from there in the memory cells.

In the case of a conventional NAND-type flash memory, the magnitude of apage of data does not exceed 512 bytes. If it is assumed that a programtime (or information storing time) of a NAND-type flash memory is about200 to 500 microseconds, and 1-byte of data is loaded on the page buffercircuit from the exterior in a period of 100 nanoseconds, it takes about50 microseconds to load 512-byte information in the page buffer circuit.

FIG. 1 shows a specific example in the prior art. FIG. 1 of the instantdocument is from U.S. Pat. No. 5,831,900 (that document's FIG. 7).Additional reference numerals have been added for the presentdiscussion.

The device of FIG. 1 teaches that data are loaded to a latch 30 from adata line IO, after page buffers 20-i are reset by the surroundingcircuitry. The data loaded to the latch are programmed to the memorycells 2-1, 2-2, 2-3, through a transistor Q4 (often by receiving anappropriate program command signal). This programming procedure isnormally used to program NAND flash memories.

This procedure, however, has a limitation. In this program operation, ifdata is to be loaded to latch 30, it will have to wait until the datathat was previously loaded finish programming in the previous programcycle. As it was described above, data loading to latch 30 progresses bybyte units (e.g. 8 bit). So, it takes a long time for data to load to apage of as many as 2048 bytes. This is because latch 30 continues tostore data until the information of the register is stored in theappropriate corresponding memory cells.

Another problem in the prior art is the copy back problem. Sometimes, acopy operation needs to be performed from a first page to a second pageof data. If it is desired to perform the copy operation after the dataof the memory cells in first page is latched to the latch circuit 30through transistor Q7, then the latched data is programmed to the secondpage through the transistor Q4. In that case, programmed data copied tothe second page are reversed, because of the latch circuit. In otherwords, 1 has become 0, and 0 has become 1. This problem is addressed inthe prior art by providing flag cells to the memory cell array, andupdating their value depending on whether the data has been inverted ornot.

FIG. 2 shows a specific example of this problem in the prior art. FIG. 2of the present document is from U.S. Pat. No. 5,996,041 (that document'sFIG. 8 and FIG. 9). Additional reference numerals have been added forthe present discussion.

In FIG. 2, copy back functions are shown. Data in the first page withinthe memory cell array is loaded to a page buffer. After that, the datais copied to another place in the array, but in inverted form. The bitto the right is the flag cell, to indicate that this data is in invertedform.

The prior art is limited as to how large the memory devices can become.For example, if it is assumed that the page buffer circuit cantemporarily store 2048-byte information, it takes about 200 microsecondsto load the 2048-byte of information when 1-byte information is loadedon a page buffer circuit by a period of 100 nanoseconds. Therefore theloading time is nearly similar to the information-storing time (or theprogram time) of 200 to 500 microseconds. Accordingly, theinformation-storing characteristic of the NAND-type flash memory isseriously affected by the loading time.

As integration of NAND-type flash memory increases, data must beprocessed in larger and larger volumes, as compared to the conventionalflash memory. And it must be processed without deterioration in theinformation-storing characteristic.

The parent application's disclosure is briefly summarized in FIGS. 22and 23 of the present application. As shown in FIGS. 22 and 23, a pagebuffer includes two registers including latches. The first register hasa first latch LATCH 1 and the second register has a second latch LATCH2. The detailed description of this structure is explained in parentU.S. patent application Ser. No. 10/013,19 1. As shown in FIG. 22, datato be programmed is loaded to the node N4 in LATCH 1 via the Data Lineduring phase F1. Next the data is transferred (or dumped) from mode N4in LATCH 1 to the node N3 of the LATCH 2 during phase F2. According tothe data state of the node N3, the data is programmed to the first pageof the memory cell array, during a program phase F3 in FIG. 23. If thedata of the node N3 is “0” (ground level and program state), then thememory cells are programmed. If on the other hand the data of the nodeN3 is “1” (Vcc level and program inhibit state), then the memory cellsare not programmed. Note that a page includes a group of memory cellsthat are controlled by one word line.

After programming, the memory cells of the page must be checked todetermine whether the memory cells have been successfully programmed.This checking procedure is referred to herein as “program verify read”,phase F4 of FIG. 23. In the program verify read procedure, if the cellto be programmed is not programmed, the state of node N3 is reset to “0”and if the cell to be programmed is programmed, the state of node N3 isreset to “1”. The cells that are not programmed must be reprogrammedaccording to the above program procedure.

If all of the cells are programmed, the node N3 is set to “1 ” duringphase F5. This concludes the procedure for the first page of the memorycell array.

During the program procedure of the first page of the memory cell array,the data of the second page are simultaneously loaded to the node N4 inthe LATCH 1. As a result, two procedures are carried out concurrently ina given page buffer.

U.S. Pat. No. 6,031,760 entitled SEMICONDUCTOR MEMORY DEVICE AND METHODOF PROGRAMMING THE SAME describes in connection with FIG. 5 thereof aprior art single-latch memory device that is typical of conventionalmemory devices. The described circuit has a single sense amplifier S/Athat includes only a single latch circuit LT.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a memory device having a page buffer in the priorart.

FIG. 2 is an illustration of a copyback operation in the prior art, andof the flag bit it render necessary because it inverts the data.

FIG. 3 is a block diagram of a semiconductor memory device madeaccording to an embodiment of the invention.

FIG. 4 is a diagram of an array scheme of the memory of FIG. 3.

FIG. 5 is a detailed circuit diagram of a page register and a Y-gatingcircuit of the device of FIG. 3.

FIG. 6 is a flowchart illustrating a programming method according to anembodiment of the present invention.

FIG. 7 is a timing diagram of signal commands for effectuating themethod of FIG. 6.

FIG. 8 is a depiction of flow of data in the circuit of FIG. 5, whilethe signal commands of FIG. 7 are being applied.

FIG. 9 is a timing diagram of signal commands for effectuating a readingmethod in the device of FIG. 3.

FIG. 10 is a depiction of flow of data in the circuit of FIG. 5, whilethe signal commands of FIG. 9 are being applied.

FIG. 11 is a flowchart illustrating a copyback method according to anembodiment of the present invention.

FIG. 12 is a timing diagram of signal commands for effectuating acopyback method according to an embodiment of the present invention inthe device of FIG. 3.

FIG. 13 is a depiction of data having been transferred from memory cellsinto a page buffer by following signal commands of a first portion ofFIG. 12.

FIG. 14 is a depiction of data having been transferred from a pagebuffer into memory cells by following signal commands of a secondportion of FIG. 12.

FIG. 15 is a flowchart illustrating an erase method according to anembodiment of the present invention.

FIG. 16 is a timing diagram of signal commands for effectuating an erasemethod in the device of FIG. 3.

FIG. 17 is a depiction of flow of data in the circuit of FIG. 5, whilethe signal commands of FIG. 16 are being applied.

FIG. 18 is a depiction of how large volumes of memory are counted fortwo alternate memory device designs.

FIG. 19 is a table illustrating various design choices for memorydevices, including the two of FIG. 18.

FIG. 20 is a block diagram illustrating the arrangement of 1 block.

FIG. 21 is a diagram illustrating a time sequence of how data would beloaded according to the present invention to achieve higher capacity.

FIG. 22 is a schematic circuit diagram representing a simplified summaryof the dual-register memory device described in detail in the parentapplication of which the present invention is a continuation in part.

FIG. 23 is a flowchart representing a simplified summary of the methodfor dual-register memory device programming described in detail in theparent application.

FIG. 24 is a schematic circuit diagram of a typical memory deviceillustrating the problem addressed by the present invention.

FIG. 25 is a graph of the distribution of voltages over a plurality ofmemory cells, and illustrates a problem addressed by the presentinvention.

FIG. 26 is a schematic circuit diagram of the invented dual-registermemory device in accordance with one embodiment of the invention.

FIG. 27 is a flowchart illustrating one embodiment of the inventeddual-register memory device programming method.

FIG. 28 is a timing diagram illustrating various signals and theirrelative timing in accordance with the invented programming method.

FIG. 29 is a graph of the distribution of voltages over a plurality ofmemory cells, and illustrates the solution proposed by the presentinvention.

DETAILED DESCRIPTION

As has been mentioned, the present invention provides semiconductormemory devices, and methods of using the same. The invention is nowdescribed in more detail.

Referring now to FIG. 3, a memory device 100 made according to theinvention is described. Memory device 100 may be a NAND flash memory.Memory device 100 has an array 110 of memory cells to store data, a PageRegister and Sense Amplifier (S/A) block 120, and a Y-gating circuit 130to gate data stored in a group of the memory cells. Page Register andS/A block 120 is coupled between memory cell array 110 and Y-gatingcircuit 130.

Page Register and S/A block 120 includes a page buffer 122. Page buffer122 includes dual registers according to the invention, as will bedescribed in more detail below.

Device 100 also includes additional components, such as X-bufferslatches and decoders, Y-buffers latches and decoders, a commandregister, a Control Logic and High Voltage Generator, and globalbuffers. They exchange data, address, and command signals as shown, andas will be understood from the description below.

Referring now to FIG. 4, a sample arrangement is shown for the array 110of the memory cells. Many bit lines are shown, alternatingly designatedBLe, BLo, with “e” representing even and “o” representing odd. Manymemory cells (M1, M2, . . . , Mm) are connected to each bit line.

A group of memory cells (e.g. M1) are controlled by a single word line(e.g. WL1). The cells in that group are called a page unit for purposesof this application.

Referring now to FIG. 5, Page Register and S/A block 120 and Y-gatingcircuit 130 are described in more detail.

Y-gating circuit 130 is between Page Register and S/A block 120 and adata line 131. Data line 131 may be for bits D0-D7.

Y-gating circuit 130 is made from two NMOS transistors 132 and 133.Transistors 132 and 133 are controlled by signals YA, YB. Signals YA, YBmay be derived from information from a column address.

Page Register and S/A block 120 includes a single page buffer 122, whichhas a sense line 125 that includes a sense node E. One or more bit linesmay be connected to page buffer 122 at node E. In the example of FIG. 5,two bit lines BLe, BLo are connected to node E.

A transistor 141 has a source connected to a corresponding bit line BLe,a drain which is connected to a node providing signal VIRPWR and a gateconnected to receive the gate control signal VBLe.

A transistor 142 has a source connected to bit line BLo, a drain whichis connected to the node providing signal VIRPWR, and a gate connectedto receive the gate control signal VBLo.

The node providing signal VIRPWR is charged at either one of a first ora second supply voltage. Accordingly, transistors 141 and 142 apply thefirst or second supply voltage to bit lines BLe and BLo, in response togate control signals VBLe and VBLo.

In addition, an NMOS transistor 143 connects the bit line BLe to node Ein response to a BLSHFe signal. An NMOS transistor 144 connects the bitline BLo to node E line in response to a BLSHFo signal.

Page buffer 122 is thus coupled to bit lines BLe, BLo through node E ofsense line 125. A PMOS transistor 148 supplies current to the bit linesBLe, BLo via sense line 125 during a read operation. The PMOS transistor148 is connected between a power supply voltage and the sense line andturns on/off according to a control signal PLOAD.

Importantly, page buffer 122 has two registers 150, 170. The prior artprovides only one such register. Both are connected to sense line 125.

Second register 150 is also known as main register 150. Main register150 includes two NMOS transistors 151, 152, two inverters 153, 154, anda PMOS transistor 155. The data is stored in main latch 156, formed byinverters 153, 154. PMOS transistor 155 forms a precharge circuit formain latch 156.

First register 170 is also called an auxiliary register 170. Auxiliaryregister 170 includes two NMOS transistors 171, 172, two inverters 173,174, and a PMOS transistor 175. The data is stored in auxiliary latch176, formed by inverters 173, 174. PMOS transistor 175 forms a prechargecircuit for auxiliary latch 176.

The dual register (made from the two registers 150, 170) of the pagebuffer 122 of the present invention provides many advantages. Functionsare performed better than in the prior art, which are found to justifyincreasing the magnitude of the page buffer circuit.

Additional structure is provided to facilitate and control exchangingdata between the two page buffer registers 150, 170, memory cell array110, and Y-gating circuit 130.

An NMOS transistor 181 controlled by a control signal PDUMP is turned onto transfer data between auxiliary register 170 and main register 150.Alternately, it is turned off to electrically isolate auxiliary register170 from main register 150. This transfer is advantageously performedover sense line 125. NMOS transistor 181 is also known as an isolationswitch.

NMOS transistors 182, 183 provide for storing information in auxiliaryregister 170. This is performed responsive to the externally inputsignals DI and nDI, respectively.

A NMOS transistor 184 connects or disconnects main register 150 to orfrom a selected one of bit lines BLe, BLo. This is performed wheninformation to be programmed is transferred to the selected one of thebit lines from main register 150.

A NMOS transistor 185 is controlled by a control signal PBDO. Transistor185 outputs information read out via the selected bit line to theexterior of page buffer 122 during a selected interval of time.

A transistor 186 is prepared for checking the program state, andprovides program pass/fail information at a node B of main register 150.

Methods of the invention are now described.

Referring now to FIG. 6, FIG. 7, FIG. 8 and also FIG. 4, programmingmethods according to the invention are described. Programming is wheredata is input in the memory cells of a device from outside the device.

In FIG. 6, a flowchart 600 is used to illustrate a programming methodaccording to an embodiment of the invention. The method of flowchart 600may also be practiced by circuit 100 of FIG. 3.

According to a box 610, first external data is passed through a Y-gatingcircuit, such as circuit 130. The first external data is passed towardsa page buffer, such as page buffer 122. It can be a single datum or manydata. It may even be a whole page of data.

According to a next box 620, the first data passed at box 610 is storedat a first register of a page buffer. The first register may beauxiliary register 170.

According to an optional next box 630, a switch may be activated toconnect the first register with a second register. The second registermay be main register 150. The switch may be NMOS transistor 181,controlled by control signal PDUMP.

According to a next box 640, the first data that is stored in the firstregister is stored at the second register.

According to an optional next box 650, the switch may be activated toisolate the first register from the second register.

According to a next box 660, the first data that is stored in the secondregister is stored at a cell of a memory cell array, which is alsocalled programming. Concurrently, second external data is received atthe first register, and stored therein. Therefore, aninformation-storing operation can be carried out without increasing theinformation-loading time.

In the embodiment of FIG. 3, the concurrent operation of box 660 is madepossible because of the isolation of the first register and the secondregister. Other methods are also possible.

Referring to FIG. 7 and FIG. 8, a programming method of the invention isdescribed in more detail. FIG. 7 shows command signals that may beapplied to the circuit of FIG. 5. The horizontal axis is divided intonine time segments, respectively labeled 1, 2, . . . , 9.

FIG. 8 shows how data is transferred in the circuit of FIG. 5, resultingfrom applying the command signals of FIG. 7. FIG. 8 should be referredto along with FIG. 7, using the same cross-referenced time segments asFIG. 7.

At a first step (time segment 1), a data line 131 is taken to a groundvoltage, and transistor 175 is turned on by PBSET signal. This is alsoknown as page buffer setting for the first page.

Afterwards (time segment 2) a node D of auxiliary latch 176 is at a highstate, and NMOS transistors 132 and 133 are turned on. Data “0” or “1 ”in data line is thus stored to auxiliary latch 176 by applying phases ofDI and nDI signals. This is also known as data loading of the firstpage, and loosely corresponds to box 610 described above.

Then (time segment 3), the stored data is transferred to sense line 125from the auxiliary register 170. This is accomplished by transitioningcontrol signal PDUMP to a logic high state. Prior to transferring thedata to main register 150, sense line 125 and node A of latch 156 areprecharged by the transistor 148 and 155 respectively.

Afterwards (time segment 4) the signals are zeroed. The process is alsocalled HV enable.

Then (time segment 5), the appropriate one of the bit lines BLe, BLo isset up, by being precharged.

Then (time segments 6 and 7), two actions happen concurrently,corresponding to box 660 above. The data to be programmed is transferredfrom main register 150 to selected bit line BLe by activating the BLSLTsignal, and from there to the memory cell. In addition, the next data tobe programmed is stored (loaded) in auxiliary register 170 from theexterior of the memory device.

Generally, the data loading operation is done by byte unit, whileprogramming operation is done by page unit. Data loading means that datais transferred from the data line to the auxiliary register 170, whileprogramming operation means that data transfers from main register 150to the memory cells in the memory cell array 110. As described above,page unit means that a plurality of memory cells are connected andcontrolled by a single word line.

Since the two actions take place concurrently, the data-storingcharacteristic is maintained even at high volumes of data. Thusimplementing the page buffer circuit with auxiliary register 170 is wellworth increasing the magnitude of the page buffer circuit.

Then (time segment 8), the read operation is verified, and (time segment9), the bit lines are precharged again for the next load/programoperation.

Referring now to FIG. 9 and FIG. 10, a read operation of the device ofFIG. 3 is described in more detail. Data is assumed to be read out fromone of the memory cells of array 110, and that gate control signals ofmemory cells to be read apply appropriate voltages to word lines.

FIG. 9 shows command signals that may be applied to the circuit of FIG.5. The horizontal axis is divided into six time segments, respectivelylabeled 1, 2, . . . , 6.

FIG. 10 shows how data is transferred in the circuit of FIG. 5,resulting from the command signals of FIG. 9. FIG. 10 should be referredto along with FIG. 9, using the same cross referenced time segments asFIG. 9.

Briefly, reading out is performed directly through main register 150,bypassing auxiliary register 170. This way, auxiliary register 170 doesnot obstruct reading data, while it facilitates data loading and dataprogramming as described above.

In order to perform a stable read operation, the bit lines BLe and BLoare first discharged through NMOS transistors 141 and 142 by zeroing theVIRPWR signal, and activating the control signals VBLe and VBLo high.(Time segment 1.)

At the same time, a PBRST signal transitions from a logic high state toa logic low state, so that a state of the main register 150 (or an inputof inverter 153) is set to a predetermined state (i.e., a logic highstate).

Afterwards, the PLOAD signal goes low, and thus PMOS load transistor 148is turned on. The control signal BLSHFe of the NMOS transistor 143 ismade to have a voltage of summing a bit line precharge voltage and athreshold voltage of the NMOS transistor 143. After precharging the bitline BLe with an appropriate voltage, the BLSHFe signal goes to a logiclow state of the ground voltage. (Time segment 2.)

A precharged voltage of the bit line is varied according to a state of aselected memory cell. For example, in the case where the selected memorycell is an off cell, the precharged voltage of the bit line continues tobe maintained. In the case where the selected memory cell is an on cell,the precharged voltage of the bit line is lowered. (Time segment 3.)

If a voltage of the BLSHFe signal is changed into an intermediatevoltage between the precharge voltage and the previous BLSHFe signallevel, a voltage on sense line 125 is maintained at the power supplyvoltage by shutting off the NMOS transistor 143 when the selected memorycell is an off cell. If not, however, a voltage on sense line 125 islowered along a bit line BLe voltage (or is synchronized with a bit lineBLe). At a midway point where the BLSHFe signal goes to a logic lowstate of the ground voltage, the PLOAD signal turns to the power supplyvoltage.

After this, a gate control signal PBLCHM of NMOS transistor 152 goes toa logic high state of the power supply voltage, and NMOS transistor 151is turned on or off according to a state of the sense line. As a result,the state of sense line 125 is stored in main register 150. (Timesegment 4.)

Then the data stored in main register 150 is transferred to the dataline via NMOS transistor 185, which is controlled by control signalsPBDO and next via Y-gating circuit 130. (Time segment 6.)

Copy-back methods according to the invention are now described. Duringthe performance of reading operation, it may become necessary to performa page copy operation by copying data read from a first page of memorycells at a first address to a second page of memory cells at a secondaddress.

Referring now to FIG. 11, a flowchart 1100 is used to illustrate acopy-back method according to an embodiment of the invention. The methodof flowchart 1100 may also be practiced by device 100 of FIG. 3.

According to a box 1110, data of a first cell is stored at a firstregister of a page buffer. This may be performed by reading out datainto the auxiliary register 170. Reading out may be performed asdescribed above.

According to a next box 1120, the data stored in the first register isstored at the second register of a page buffer. This may be performed bytransferring the read out data between the auxiliary register 170 andthe main register 150. The transfer may optionally involve activating aswitch to connect the first register with the second register.

According to a next box 1130, the data of the second register is storedat a second cell of the memory cell array. This may be performed as aprogramming operation, as described above.

Referring now to FIG. 12, FIG. 13, FIG. 14, a copy-back operation of thedevice of FIG. 3 is described in more detail. Data is assumed to be readout from original memory cells of array 110 into page buffer 122, andcopied back there, into different cells.

FIG. 12 shows command signals that may be applied to the circuit of FIG.5. The horizontal axis is divided into eleven time segments,respectively labeled 1, 2, . . . , 11.

The data is first read out from the cells to the page buffer. It will berecognized that the signal commands in the first four time segments 1,2, 3, are substantially the same as in those of FIG. 10, except thatdata is read into auxiliary register 170, instead of main register 150.

Referring to FIG. 13, the data read out into the page buffer is shown. Ablank space is also shown, where the prior art of FIG. 2 required anadditional indicator bit to indicate the polarity (inverted or not) ofthe stored data.

Returning to FIG. 12, the data is then transferred from auxiliaryregister 170 into main register 150 of the page buffer. This takes placeduring time segments 5, 6.

Then the data is programmed from main register 150 into other cells ofthe memory, during time segments 7, 8, 9, 10, 11. It will be recognizedthat the signal commands during time segments 5-11 are substantially thesame as in those of FIG. 8.

Referring to FIG. 14, the reprogrammed data is shown. It will beappreciated that the data is stored in the different cells according tothe invention without being inverted from how they were stored in theoriginal cells. Accordingly, there is no need to include the indicatorbit of FIG. 2, which further saves space.

Erase methods according to the invention are now discussed. Erasinggenerally dumps data. In a flash memory, the threshold voltage goes to avalue between −1V and −3V by applying a high voltage to the memorycells. Data in registers is dumped.

Referring now to FIG. 15, a flowchart 1500 is used to illustrate averify read operation after erasing according to another embodiment ofthe invention. The method of flowchart 1500 may also be practiced bydevice 100 of FIG. 3.

According to a box 1510, data of first memory cell is dumped through afirst register of a page buffer.

According to another box 1520, data stored in the first register of thepage buffer circuit is dumped through a second register.

According to an optional box 1530, data stored in the first register ischecked pass or fail of the memory cell state by transistor 186.

Referring now to FIG. 16 and FIG. 17, an erase method is described forthe device of FIG. 3. FIG. 16 shows command signals that may be appliedto the circuit of FIG. 5. The horizontal axis is divided into seven timesegments, respectively labeled 1, 2, . . . , 7.

FIG. 17 shows how data is erased in the circuit of FIG. 5, resultingfrom applying the command signals of FIG. 16. FIG. 17 should be referredto along with FIG. 16, using the same cross-referenced time segments asFIG. 16.

In time segments 1 and 2, an erase execution command is received. Intime segment 3, bit lines BLe, BLo are grounded for discharge. In timesegment 4, a Verify Read operation takes place for a first cell. In timesegment 5, a Verify Read operation takes place for a second cell.

In time segment 6, data is dumped through the first register. The dataincludes data of a memory cell, and also data from main register 150 andsupplemental register 170 of the page buffer. In time segment 7, a wiredOR operation takes place, and data is dumped from node B of mainregister 150.

The invention offers the advantage that, even if the size of the page isincreased, the program time (or the information-storing time) of thememory is increased slightly or not at all. In addition, a time forloading information on the page buffer circuit is increased inproportion to the increased magnitude of the page.

Referring to FIG. 18, FIG. 19, FIG. 20, FIG. 21, examples are discussedof handling large volumes of data in memories. Efficiencies of theinvention are thus illustrated.

FIG. 18 is a depiction of how large volumes of memory are counted forthe capacity of a memory device, for two cases, namely A and B.

A three dimensional box depicts the total memory capacity of a device.It can be thought of as a stack of blocks, and each block is a stack ofpages. Each page (and also each block) is one byte (IB) wide. A byteequals eight bits, namely I/O 0-I/O7.

In case A, one page is (512+16) 528 B long. Assuming blocks of 32 pages,a capacity of 2048 blocks will yield a device of 264 Mbit.

In case B, enabled by the present invention, one page is (2048+64) 2112B long. Assuming blocks of sixty-four pages, a capacity of 1024 blockswill yield a device of 1 Gbit.

FIG. 19 displays various design choices for memory devices, includingdevices A and B of FIG. 18.

FIG. 20 illustrates how a block can be reconfigured to sixty-four pages(such as for device B of FIG. 18) from thirty-two pages (such as fordevice A of FIG. 18) by designating successive pages of data as “even”and “odd”.

The invention attains faster loading times than the prior art. This isillustrated by examples. Assuming:

T1=1 B loading time=0.1 μs

F2=1 page (for two cases of 528 B and 2112 B)

T3=program time=200 μs

F4=1 block (here thirty-two pages)

Then the time required by the device of the prior art for the sequenceof data loading, program, data loading, program, etc. requiresTotal time (prior art)=[(T1×F2)+T3]×F4   Equation (1)

This yields 8,089.6 μs for a device of 528 B and 13158.4 μs for a deviceof 2112 B. Accordingly, it is not possible to store information of largevolume into the page buffer in a show time (the information-storingcharacteristic deteriorates).

Referring to FIG. 21, data would be loaded and programmed according tothe present invention more efficiently. The total time required would beTotal time (present invention)=(T1×F2)+(T3×F4)   Equation (2)

This, for a device of 2112 B, yields 6611.2 μs, which is about half ofthe comparable time of Equation 1. This means that a page buffer circuitof a large volume (e.g., over 2048 B) may now be used.

FIGS. 24 through 29 relate to another embodiment of the invention, to bedescribed in detail below.

FIG. 24 shows a memory cell array 100 in a NAND flash memory device. Thememory cell array has a plurality of strings that comprise a pluralityof memory cells. Each of the string is connected to one bit line. Thestrings are connected to a common source line CSL in parallel. Thecommon source line CSL is connected to ground.

In the NAND flash memory device, al of the memory cells that areconnected to one word line are simultaneously programmed. In otherwords, if the word line WL1 is enabled, all of the memory cells MC1 areprogrammed according to the state of the bit line. If the bit line stateis “0”, then the memory cells are programmed. If the state of the bitline is “1”, then the memory cells are not programmed.

Afterwards, during the program verify procedure, the state of the memorycells is latched in the data node (N3 of LATCH 2 in FIG. 26).

When the bit line state is “0”, all of the memory cells are notprogrammed in the first program step.

Normally, the cells are successfully programmed after several steps ofthe program procedure. Because the coupling ratios of the memory cellsare different from one another according to the vagaries of thesemiconductor manufacturing process, even though the state of the bitline is a program state “0”, all of the memory cells to be programmedare not necessarily programmed during a single cycle or step of theprogram procedure. In general, before starting the program procedure,all of the memory cells in a NAND flash memory are erased. Accordingly,all of the memory cells have a negative threshold voltage. After severalprogram steps in the first page, all of the memory cells go to apositive threshold voltage above the verify voltage. In a given pagecontaining plural memory cells, if the first page has finished the firstprogram step then during the program verify procedure all of the memorycells are checked whether the threshold voltage of the memory cells isbelow the verify voltage or not. The verify voltage is shown in FIG. 25.At that time, even though a portion of the memory cells have beensuccessfully programmed (to “0”), nevertheless most of the memory cellstypically are below the range of the verify voltage for theabove-described reason.

Referring still to FIG. 24, during the program verify procedure, thevoltage level of a common source line CSL rises because of resistors R0,R1, R2, . . . Rm and currents Ic0, Ic1 Ic2, . . . Icm. This of course isderived from Ohm's law (V=IR). Those of skill in the art will appreciatethat the resistors R0, R1, R2, . . . Rm represent parasitic resistancesof the common source line and the currents Ic0, Ic1, Ic2, . . . Icmrepresent the currents that flow from each bit line to the common sourceline. Such currents flow through the cells that remain in erased stateor that are not sufficiently programmed.

As a result, the voltage level of the common source line CSL risesbecause of the current flowing through the strings. The fluctuation ofthe voltage level of the common source is referred to as CSL noise.

This phenomenon occurs more readily after the first program step becauseof the memory device's condition. But after several program steps, thephenomenon is minimized because the current that flows to the memorycells is minimal.

Referring now to FIG. 25, because of the CSL noise, during the programverify procedure, the LATCH 2 sets the node 3 to a programmed state “1”,even though the threshold voltage of the memory cell is actually belowthe level of the verity voltage. As a result, the memory cell that isnot sufficiently programmed is falsely and misleadingly indicated as asufficiently (successfully) programmed cell.

For example, if the memory cell MC0 has a threshold voltage of 0.3Vafter the first program, and the level of the CSL is 0.7V because of theCSL noise, the threshold voltage of the memory cell MC0 becomes 0.7Vduring the program verify procedure.

If the verify voltage is 0.7V, the memory cell is indicated as aprogrammed memory cell in the page buffer. Accordingly, the node N3 ofthe LATCH 2 goes to “1”.

In other words, even though the memory cell (MC0 in FIG. 24) is notsufficiently programmed, the node N3 of the LATCH 2 is in the high state“1”. If the memory cell is programmed in the second step, because thenode N3 of the LATCH 2 remains in the state “1”, the threshold voltageof the memory cell MC0 having a 0.3V threshold voltage is not changed.

One object of this invention is to solve this problem.

Another object of this invention is that the memory cell not to beprogrammed sustains a program inhibit state and the memory cell to beprogrammed cell is reprogrammed even though the memory cell is falselyindicated as having achieved a programmed state during the programverify procedure.

FIG. 26 depicts the present invention in schematic circuit form. FromFIG. 26 it may be seen that the present invention comprises a storingcircuit and a restoring circuit not shown in the embodiments of theinvention described in parent U.S. application Ser. No. 10/013,191.

By reference to FIGS. 26 and 27, the present invention will beexplained.

In FIG. 26, a page buffer comprises a first sense amplifier 1, a secondsense amplifier 2, a pass/fail check circuit, a storing circuit and arestoring circuit. Those of skill in the art will appreciate that thesense amplifier (1 or 2) is referred to as a register in the parentpatent application.

In step F1, the data to be programmed and the data to be programinhibited are loaded to the node N4 in one data register LATCH 1. Thedata to be programmed is “0” (GND) and the data to be program inhibitedis “1” (VDD).

In step F2 (in FIG. 27), the data “0” and “1” are dumped to the nodeN_DATA. Before step F2, the node N_DATA is pre-charged to VDD levelaccording to a PRE signal.

In step F3, the data in the node N4 is dumped to the node N3 of anotherdata register LATCH2 through the transistor TR12. The phase of the datain the node N3 is the same as the phase of the data in the node N4 andis the inverse phase of the data in the node N_DATA in the storingcircuit.

In step F4, the memory cells are programmed according to the state ofthe node N3 of the other register LATCH2. If the state of the node N3 is“0”, then the memory cell is programmed. If the state of the node N3 is“1”, then the memory cell is not programmed. The program state meansthat the threshold voltage of the memory goes to a level above theverify voltage, wherein the verify voltage has an intermediate levelbetween the threshold voltage of a programmed memory cell and that of anerased memory cell.

In step F5, the node N3 is restored according to the state of thestoring circuit. If the state of the node N_DATA is “1”, then the nodeN3 is reset to “O”. If the state of the node N_DATA is “0”, then thenode N3 retains the previous data.

In step F6, the program verify read procedure is executed. In the firstprogram verify read step, the memory cell that is not sufficientlyprogrammed is indicated as being in a programmed state in the LATCH 2.But the memory cell is indicated as a cell that is not programmedbecause the CSL noise is reduced after several program steps. Becausethe node N3 is reset to “0” according to the state of the storingcircuit, the memory cell that is insufficiently programmed is programmedduring the next program step.

In step F7, the state of the node N3 of LATCH2 is checked in thepass/fail check circuit. If the state of the node N3 is “1”, then theprogram procedure is finished. If not, then the procedure returns tostep F4.

FIG. 28 is a timing diagram of the invented programming and verifyingmethod. The steps F1 through F7 are represented along the horizontalaxis, while the various control and data signals are represented alongthe vertical axis. The control signals include X-Decoder Signals SSL;W/L (Sel.) (selected word line); W/L (Unsel.) (unselected wordline);GSL; CSL (common source line). They also include Page Buffer signalsVIRPWR (power supply voltage); VBLe (even bit line voltage); VBLo (oddbit line voltage); BLSI-IFe (even bit line shift voltage); BL,SHFo (oddbit line shift voltage); PBLCHM (gate control); PBLCHC; PLOAD; PBset;PDUMP1; BLSLT (selected bit line); DI (data input); nDI (inverse datainput); PRE (precharge); RESET; PDUMP2 and DATA LINE. These signals willbe understood in large part to be conventional or understood from thedisclosure of the parent application.

As may be seen from FIG. 28, in accordance with the invention, PDUMP2(during phase F2) precedes PDUMP1 (during phase F3) so that the previousstate of node N3 of LATCH 2 is temporarily stored for restoration ofnode N3 in the case the bit must be programmed again by returning whenneeded to step F4, as described above.

Table 1 below illustrates typical voltages for the program and verifymodes of programming a memory device of the type described herein. TABLE1 WL W BL BL (selected) (unselected) (program) (inhibit) PROGRAM 18 V 12V 0 V Vcc VERIFY 1 V 4.5 V 0.8 V 0.8 V

The word line voltage step-up, with the programming voltages and steps,proceeds as follows:

15.5V→VERIFY→16V→VERIFY→16.5V→ . . . (and so on)

In accordance with one embodiment of the invention, the maximum step-upcount (number of cycles) is twelve and the step-up voltage increment is0.5V/step. Those of skill in the art will appreciate that alternativemaximum step-up counts and/or alternative step-up voltage increments arecontemplated, and are within the spirit and scope of the invention.Typically, programming is completed within in five or six steps so thatthe maximum count is not reached.

Finally, FIG. 29 is a graph showing the distribution of voltages acrossa plurality of memory cells after programming in accordance with theinvention. It may be seen by contrast to FIG. 25 that, in accordancewith the invention, the number of bits successfully programmed risessignificantly by effectively pushing the programming of all orsubstantially all data “0”-programmed cells to a higher thresholdvoltage that is above their verify voltages. This is illustrated by thelack of any overlap in FIG. 29 of the data “0” programming of all bits(represented by the bell curve on the right side of the graph) and theverify voltage level (represented by a vertical dashed line).

A person skilled in the art will be able to practice the presentinvention in view of the description present in this document, which isto be taken as a whole. Numerous details have been set forth in order toprovide a more thorough understanding of the invention. In otherinstances, well-known features have not been described in detail inorder not to obscure unnecessarily the invention.

While the invention has been disclosed in its preferred embodiments, thespecific embodiments as disclosed and illustrated herein are not to beconsidered in a limiting sense. Indeed, it should be readily apparent tothose skilled in the art in view of the present description that theinvention may be modified in numerous ways. The inventor regards thesubject matter of the invention to include all combinations andsub-combinations of the various elements, features, functions and/orproperties disclosed herein.

The following claims define certain combinations and sub-combinations,which are regarded as novel and non-obvious. Additional claims for othercombinations and sub-combinations of features, functions, elementsand/or properties may be presented in this or a related document.

1. A read method for a nonvolatile memory device which includes a pagebuffer having a first register and a second register, comprising:initializing the second register after discharging a bit line;precharging the bit line with a bit line voltage; isolating the bit linefrom the second register; and connecting the second register and the bitline to latch data in a selected memory cell connected to the bit line.2. The read method of claim 1, wherein during a read operation via thesecond register, the first register is not associated with the readoperation.
 3. The read method of claim 1, wherein the bit line and thesecond register are selectively connected to each other via atransistor.
 4. The read method of claim 3, wherein the control signal isapplied to the transistor, the control signal having a first voltageduring precharging of the bit line, a ground voltage during isolating ofthe bit line from the second register, and a second voltage lower thanthe first voltage and higher than the ground voltage during connectingof the bit line and the second register.
 5. The read method of claim 4,wherein the first voltage is a sum of the bit line voltage and athreshold voltage of the transistor.
 6. The read method of claim 1,further comprising: outputting the latched data in the second registerto a data line.
 7. The read method of claim 6, wherein the latched datais transferred to the data line via a transistor and a Y-gating circuit.8. The read method of claim 1, wherein the first register is associatedwith a program operation and a verify read operation.